Blowing agent introduction into hopper of polymer foam processing

ABSTRACT

Methods and systems that include introducing blowing agent into a hopper of a polymeric foam system.

FIELD

The present invention relates generally to polymer foam processing and,more particularly, to methods and systems that include introducingblowing agent into a hopper of a polymeric foam processing system.

BACKGROUND

Polymeric foams include a plurality of voids, also called cells, in apolymer matrix. Polymeric foams are processed using a variety oftechniques. For example, polymeric foams can be processed by injecting aphysical blowing agent into the polymeric material within an extruder.For instance, many conventional systems inject blowing agent through ablowing agent port in the barrel of the extruder into a fluid stream ofpolymeric material within the extruder. The blowing agent may be mixedwith the polymeric material to form a solution within the extruder. Thesolution may be, for example, injected into a mold to form an injectionmolded polymeric foam article. Such conventional systems may requiremodifications to standard extruder equipment (e.g., to extend length ofthe barrel to ensure sufficient mixing, to form a blowing agent port,etc.) and/or utilize relatively expensive equipment (e.g., blowing agentmass flow meter) to control the flow and introduction of blowing agentinto the extruder.

Other prior art systems have attempted to simplify the design of polymerfoam processing systems and reduce their costs. For example, certainsystems have introduced physical blowing agent into a hopper of theextruder. Such systems generally have not controlled the amount ofblowing agent introduced into the polymeric material. For certain foamprocesses, such an approach to blowing agent delivery may be adequate.However, in other processes that require relatively precise control overblowing agent delivery, such an approach may not be adequate.

Accordingly, there is a need for new blowing agent introductiontechniques that may be used with polymer foam processes.

SUMMARY

Methods and systems including introducing blowing agent into a hopper ofa polymeric foam system are described.

In one aspect, a system is provided. The system comprises an extruderincluding a screw configured to rotate in a barrel to convey a mixtureof polymeric material and blowing agent in a downstream direction in apolymer processing space defined between the screw and the barrel. Thesystem further comprises a mold connected to an outlet of the extruder.The screw is configured to periodically move in a downstream directionin the barrel to inject a shot of the mixture of polymeric material andblowing agent into the mold. The system further comprises a hopperconfigured to hold polymeric material pellets and blowing agent in achamber volume. The hopper includes at least one inlet connectable to asource of the blowing agent. The hopper includes an outlet connectableto the polymer processing space in the extruder. The system furthercomprises a pressure regulator constructed and arranged to control thepressure of blowing agent supplied to the hopper. The system furthercomprises at least one processor and at least one storage medium havingencoded thereon executable instructions that, when executed by the atleast one processor, cause the at least one processor to carry out amethod which comprises controlling a pressure of the blowing agentsupplied to the hopper to a desired pressure using the pressureregulator based, at least in part, on the desired weight percentage ofblowing agent in the shot of the mixture of polymeric material andblowing agent.

In one aspect, a method is provided. The method comprises providing ahopper configured to hold polymeric material pellets and supplyingblowing agent to the hopper at a desired blowing agent pressure based,at least in part, on a desired weight percentage of blowing agent in theshot. The method further comprises supplying blowing agent and thepolymeric material pellets to an inlet of an extruder including a screwconfigured to rotate in a barrel. The method further comprises conveyinga mixture of polymeric material and the blowing agent in a downstreamdirection in the extruder and accumulating a shot of the mixture ofpolymeric material and blowing agent. The method further comprisesinjecting the shot into a mold to form a molded polymeric foam article.

In one aspect, a system is provided. The system includes an extruderincluding a screw configured to rotate in a barrel to convey a mixtureof polymeric material and blowing agent in a downstream direction in apolymer processing space defined between the screw and the barrel. Thesystem further includes a mold connected to an outlet of the extruder.The screw is configured to periodically move in a downstream directionin the barrel to inject a shot of the mixture of polymeric material andblowing agent into the mold. The system further comprises a blowingagent introduction system including a source of blowing agent and apressure regulator. The system further comprises a hopper including achamber volume having a port fluidly connected to the source of blowingagent. The hopper is configured to hold polymeric material pellets andblowing agent in a chamber volume. The hopper includes a first outletconfigured to supply polymeric material pellets and blowing agent to thepolymer processing space in the extruder. The system is configured torecycle blowing agent in the chamber volume to a location in the blowingagent introduction system upstream of the pressure regulator.

In one aspect, a system is provided. The system includes an extruderincluding a screw configured to rotate in a barrel to convey a mixtureof polymeric material and blowing agent in a downstream direction in apolymer processing space defined between the screw and the barrel. Thesystem further includes a mold connected to an outlet of the extruder.The screw is configured to periodically move in a downstream directionin the barrel to inject a shot of the mixture of polymeric material andblowing agent into the mold. The system further includes a blowing agentintroduction system including a source of blowing agent and a hopperincluding at least a first chamber volume and a second chamber volume.At least one of the first or the second chamber volumes are configuredto connect to the source of blowing agent and to hold polymeric materialpellets and blowing agent.

Other aspects and features will become apparent from the followingdetailed description of the invention when considered in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a polymer foam processing systemaccording to an embodiment.

FIG. 2 schematically illustrates a multi-chamber hopper assemblyaccording to an embodiment.

FIG. 3 schematically illustrates a computing device suitable for use inconnection with a polymer foam processing system according to anembodiment.

DETAILED DESCRIPTION

Methods and systems that include introducing blowing agent into a hopperof a polymeric foam system are described. The methods may utilize acontrol system that enhances control over the amount of physical blowingagent (e.g., nitrogen, carbon dioxide) introduced into the polymericmaterial being processed by the system. As described further below, thesystem controls the amount of blowing agent introduced, in part, bycontrolling the pressure of blowing agent supplied to the hopper to adesired amount. The desired pressure may be based, at least in part, onthe desired weight percentage of blowing agent in the polymeric materialbeing processed. As described further below, the methods may determinethe desired blowing agent pressure from a variety of additional inputswhich may relate to the blowing agent (e.g., blowing agent type),desired article characteristics (e.g., article weight), polymercharacteristics (e.g., polymer type, polymer pellet bulk density) andequipment design (e.g., hopper chamber volume). The methods and systemsare particularly well suited for processes that produce injection moldedpolymeric foam articles. In some embodiments, the methods and systemsare designed to recycle blowing agent to reduce the amount of unusedblowing agent. The methods and systems may also include a multi-chamberhopper design that facilitates re-filling the chamber(s) with polymericmaterial pellets, for example, when a chamber is empty.

Referring to FIG. 1, a blowing agent introduction system 10 is used todeliver physical blowing agent to a polymer processing system 12. Inthis embodiment, system 12 is an injection molding system that includesan extruder 14 and a mold 16. A hopper 18 provides polymeric material(e.g., in the form of pellets) to the extruder. As described furtherbelow, the hopper includes a chamber volume in which pellets of thepolymeric material are contained. According to the methods describedherein, the blowing agent introduction system supplies blowing agent tothe chamber volume in the hopper which can be supplied, along withpolymeric material, to the extruder. As described further below, theblowing agent introduction system controls the pressure of the blowingagent supplied to the hopper based on the desired amount of blowingagent (i.e., blowing agent concentration) in the polymeric material and,in some cases, additional parameters.

The extruder includes a screw 20 designed to rotate within a barrel 22to plasticate polymeric material. Heat (e.g., provided by heaters on theextruder barrel) and shear forces (e.g., provided by the rotating screw)act to melt the polymeric material to form a fluid polymeric streamwhich is mixed with the blowing agent supplied from the hopper. Themixture is conveyed in a downstream direction 24 in the extruder barrelby rotation of the screw. In some embodiments, the mixture is asingle-phase solution with the physical blowing agent being dissolved inthe polymeric material prior to injection into the mold. In theillustrated embodiment, a valve 29 is arranged between the outlet of theextruder and the inlet of the mold. A shot of the mixture (e.g.,single-phase solution) may be accumulated downstream of the screw withinthe extruder causing the screw to retract in an upstream directionwithin the barrel. When suitable conditions have been reached (e.g.,after a predetermined time period, at a predetermined screw position,etc.), the screw stops retracting and rotating to end the plasticationperiod of the molding cycle. During the injection period of the moldingcycle, the screw may be forced downstream within the barrel to injectthe mixture into a cavity of the mold when valve 29 opens. The mixtureis subjected to a pressure drop during injection which nucleates a largenumber of cells and a polymer foam article is formed in the mold. Thescrew may begin to rotate once again to begin another molding cycle. Themethod is typically repeated to produce multiple polymeric foamarticles.

It should be understood that the polymer foam processing system mayinclude a number of conventional components not illustrated in thefigure. Though the blowing agent introduction system is illustrated asbeing used in conjunction with an injection molding system, it should beunderstood that the blowing agent introduction system may be used inconjunction with any other polymer processing system into which blowingagent is introduced including blow molding systems.

The blowing agent introduction system includes a blowing agent source 26connectable to one or more port(s) 28 that are connectable to a chambervolume in the hopper. Conduit 36 is used to connect various componentsof the introduction system and to provide a pathway from the source tothe blowing agent port(s). Upstream of the hopper, the blowing agentintroduction system includes a pressure regulator 38 which, as describedfurther herein, may be used to set the pressure of blowing agentsupplied to the hopper at a desired level. In some embodiments, theblowing agent introduction system may include an accumulator 47connected to an interchangeable bottle of blowing agent. In someembodiments, such as when a bottle does not supply blowing agent at asufficiently high pressure, a pump may be connected to increase and/ormaintain pressure of blowing agent in the introduction system. A controlsystem 44 of the blowing agent introduction system may receive one ormore inputs (e.g., relating to the desired amount of blowing agentintroduced into the polymeric material which may be selected by anoperator) and can provide output(s) to control the pressure regulator tosupply a desired blowing agent pressure to the hopper. It should beunderstood that the blowing agent introduction system may include otherstandard components such as valves which may be used to selectivelycontrol blowing agent flow therepast. As described further below, theblowing agent introduction system may be configured to recycle residualblowing agent remaining in the hopper.

The control system may be any of the type known in the art such as acomputing device, as described further below. As described above, thecontrol system is capable of receiving input signals (e.g., from a user,from other components of the polymer processing system) and sendingappropriate output signals (e.g., to components of the blowing agentintroduction system such as the pressure regulator and/or the polymerprocessing system).

As noted above, techniques described herein involve supplying blowingagent to the hopper at a desired pressure. Such an approach may be usedto supply a desired amount of blowing agent into the polymeric material(e.g., desired weight percentage of blowing agent material in the shotof polymeric material injected into the mold) which, for example, may beselected (e.g., as a value that is inputted to the system) by a user. Asdescribed further below, the desired pressure may be determined from anumber of parameters in addition to the desired weight percentage ofblowing agent in the polymeric material.

In some embodiments, the parameters may include characteristics relatingto the equipment design. For example, the hopper chamber volume may beused as a parameter.

In some embodiments, the parameters may include characteristics relatingto the polymeric material. For example, the type of polymer (e.g., resintype such as polypropylene, polyethylene, etc.), weight of polymericmaterial and/or polymeric material density may be used as parameter(s).

In some embodiments, the parameters may include characteristics relatingto the injected molded article. For example, the weight (e.g., mass ofpolymeric material) of the injection molded article may be used.

In some embodiments, the parameters may include characteristics relatingto the blowing agent. For example, the type of blowing agent (e.g.,nitrogen, carbon dioxide) may be a parameter that is used in addition tothe desired weight percentage of blowing agent in the polymeric materialnoted above.

In some embodiments, one aspect of determining the desired blowing agentpressure supplied to the hopper involves a step of determining thevolume of blowing agent in the chamber in the hopper.

In some embodiments, one aspect of determining the desired blowing agentpressure supplied to the hopper involves a step of determining theamount of blowing agent that leaks out of the chamber volume of thehopper.

In some embodiments, one aspect of determining the desired blowing agentpressure supplied to the hopper involves a step of determining themaximum number of shots that may be achieved when using a hopper havinga certain chamber volume.

The systems and methods described herein system may be used to introduceblowing agent into polymeric material within the extruder over a widerange of desired amounts. The desired blowing agent amount depends uponthe particular process and is generally less than about 10% by weight ofpolymeric material and blowing agent. In many embodiments, the blowingagent level is less than about 5%, in others, less than about 3%, inothers less than about 1%, in others less than about 0.5%, and stillothers less than about 0.1%, or even lower by weight of polymericmaterial and blowing agent mixture.

During an illustrative process, the source provides blowing agent to theintroduction system. The source may supply any type of physical blowingagent known to those of ordinary skill in the art including nitrogen,carbon dioxide, hydrocarbons, chlorofluorocarbons, noble gases and thelike or mixtures thereof. The blowing agent may be supplied in anyflowable physical state such as a gas, a liquid, or a supercriticalfluid. According to one preferred embodiment, the source providesnitrogen as a blowing agent. In another preferred embodiment, the sourceprovides carbon dioxide as a blowing agent. In certain embodiments,solely carbon dioxide or nitrogen is used. Blowing agents that are inthe supercritical fluid state after injection into the extruder,(optionally, before injection as well) and in particular supercriticalcarbon dioxide and supercritical nitrogen, are preferred in certainembodiments.

In some embodiments and as illustrated, the system is designed torecycle unused blowing agent. For example, the system may be configuredto recycle residual blowing agent remaining in the chamber volume in thehopper after the polymeric material pellets in the hopper have beensupplied to the extruder. In some cases, the residual blowing agent isremoved from the hopper (e.g., via a port in the chamber volume) andre-circulated back into the blowing agent introduction system so that itmay be used again. As shown in FIG. 1, the blowing agent may bere-circulated via conduit 46. Conduit 46, for example, re-introduces theblowing agent into the blowing agent introduction system at a positionupstream of the pressure regulator. In some embodiments, the blowingagent is re-introduced into an accumulator of the blowing agentintroduction system. In some embodiments, re-circulated blowing agentmay be re-introduced into the chamber volume of the hopper that containsunused polymeric material pellets.

In some embodiments, the polymer foam processing system includes ahopper having multiple chambers. For example, FIG. 2 schematicallyillustrates a multi-chamber hopper assembly 100 according to anembodiment. The multi-chamber hopper assembly, as shown, includes afirst chamber 102 and a second chamber 104. The first and secondchambers are connected to a source of polymeric material pellets (notshown). In some cases, and as shown, the hopper assembly includes aloader 106 which is configured to contain polymeric material pelletsupstream of the hopper chambers. The assembly may include respectiveshut-off valves 108 arranged between the loader and inlets to thechambers which may be controlled to permit or prevent polymeric materialpellets from entering the chambers. The assembly may also includeblowing agent inlets 112 which are fluidly connected to the blowingagent source. Shut-off valves may be associated with inlets 112 topermit or prevent blowing agent from flowing into the chambers. Shut-offvalves 114 may also be positioned at respective outlets 115, 117 of thechambers. In the illustrative embodiment, the outlets are connected to athird chamber 116. During use, one of outlet shut-off valves is open topermit supply of polymeric material pellets (and blowing agent) to thethird chamber, while the other of the outlet shut-off valves is closedto prevent supply of polymeric material pellets and blowing agent to thethird chamber. When all of the polymeric material pellets are used fromone of the chambers the appropriate outlet valve is closed and the otheroutlet valve is opened to enable the other chamber to supply polymericmaterial pellets to the third chamber. The third chamber includes anoutlet 118 that is connected to the polymer processing space so thatpolymeric material pellets and blowing agent may be supplied to theextruder. As illustrated, the third chamber also includes a blowingagent inlet that is fluidly connected to the blowing agent source.

Pressure may be maintained within the chambers that are supplying thepolymeric material pellets and blowing agent to the extruder and reducedin the other chamber. That is, when the outlet of the first chamber isopen and the outlet of the second chamber is closed, pressure may bemaintained in the first chamber and the third chamber and may be reducedin the second chamber (e.g., to atmosphere, for example, to enablepolymeric material pellets to be added to the second chamber).Similarly, when the outlet of the second chamber is open and the outletof the first chamber is closed, pressure may be maintained in the secondchamber and the third chamber and may be reduced in the first chamber(e.g., to atmosphere, for example, to enable polymeric material pelletsto be added to the second chamber). When pressure is reduced in one ormore chambers, it may be accomplished by releasing the blowing agentfrom the chamber. In some cases, the released blowing agent may berecycled as described above.

It should be understood that the multi-chamber hopper assembly mayinclude additional components that are not illustrated. In someembodiments, blowing agent is not supplied to first chamber 102 orsecond chamber 104 and is only supplied to third chamber. In suchembodiments, the first chamber and second chamber may not includeblowing agent ports, while the third chamber may include a blowing agentport.

In general, the blowing agent introduction system may be used with thepolymer processing system to produce any type of polymeric foammaterial. In some embodiments, the blowing agent introduction system maybe used to introduce blowing agent into a polymer processing system thatproduces microcellular polymeric foam materials. In some embodiments,the microcellular polymeric foam materials produced may have an averagecell size of less than 100 microns. It should be understood thatpolymeric foam materials having larger cell sizes may also be formedusing the systems and methods described herein.

Techniques operating according to the principles described herein may beimplemented in any suitable manner. Included in the discussion above area series of flow charts showing the steps and acts of various processesthat are described herein. The processing and decision blocks of theflow charts above represent steps and acts that may be included inalgorithms that carry out these various processes. Algorithms derivedfrom these processes may be implemented as software integrated with anddirecting the operation of one or more single- or multi-purposeprocessors, may be implemented as functionally-equivalent circuits suchas a Digital Signal Processing (DSP) circuit or an Application-SpecificIntegrated Circuit (ASIC), or may be implemented in any other suitablemanner. It should be appreciated that the flow charts included herein donot depict the syntax or operation of any particular circuit or of anyparticular programming language or type of programming language. Rather,the flow charts illustrate the functional information one skilled in theart may use to fabricate circuits or to implement computer softwarealgorithms to perform the processing of a particular apparatus carryingout the types of techniques described herein. It should also beappreciated that, unless otherwise indicated herein, the particularsequence of steps and/or acts described in each flow chart is merelyillustrative of the algorithms that may be implemented and can be variedin implementations and embodiments of the principles described herein.

Accordingly, in some embodiments, the techniques described herein may beembodied in computer-executable instructions implemented as software,including as application software, system software, firmware,middleware, embedded code, or any other suitable type of computer code.Such computer-executable instructions may be written using any of anumber of suitable programming languages and/or programming or scriptingtools, and also may be compiled as executable machine language code orintermediate code that is executed on a framework or virtual machine.

When techniques described herein are embodied as computer-executableinstructions, these computer-executable instructions may be implementedin any suitable manner, including as a number of functional facilities,each providing one or more operations to complete execution ofalgorithms operating according to these techniques. A “functionalfacility,” however instantiated, is a structural component of a computersystem that, when integrated with and executed by one or more computers,causes the one or more computers to perform a specific operational role.A functional facility may be a portion of or an entire software element.For example, a functional facility may be implemented as a function of aprocess, or as a discrete process, or as any other suitable unit ofprocessing. If techniques described herein are implemented as multiplefunctional facilities, each functional facility may be implemented inits own way; all need not be implemented the same way. Additionally,these functional facilities may be executed in parallel and/or serially,as appropriate, and may pass information between one another using ashared memory on the computer(s) on which they are executing, using amessage passing protocol, or in any other suitable way.

Generally, functional facilities include routines, programs, objects,components, data structures, etc. that perform particular tasks orimplement particular abstract data types. Typically, the functionalityof the functional facilities may be combined or distributed as desiredin the systems in which they operate. In some implementations, one ormore functional facilities carrying out techniques herein may togetherform a complete software package. These functional facilities may, inalternative embodiments, be adapted to interact with other, unrelatedfunctional facilities and/or processes, to implement a software programapplication. In other implementations, the functional facilities may beadapted to interact with other functional facilities in such a way asform an operating system. In other words, in some implementations, thefunctional facilities may be implemented alternatively as a portion ofor outside of an operating system.

Some exemplary functional facilities have been described herein forcarrying out one or more tasks. It should be appreciated, though, thatthe functional facilities and division of tasks described is merelyillustrative of the type of functional facilities that may implement theexemplary techniques described herein, and that embodiments are notlimited to being implemented in any specific number, division, or typeof functional facilities. In some implementations, all functionality maybe implemented in a single functional facility. It should also beappreciated that, in some implementations, some of the functionalfacilities described herein may be implemented together with orseparately from others (i.e., as a single unit or separate units), orsome of these functional facilities may not be implemented.

Computer-executable instructions implementing the techniques describedherein (when implemented as one or more functional facilities or in anyother manner) may, in some embodiments, be encoded on one or morecomputer-readable media to provide functionality to the media.Computer-readable media include magnetic media such as a hard diskdrive, optical media such as a Compact Disk (CD) or a Digital VersatileDisk (DVD), a persistent or non-persistent solid-state memory (e.g.,Flash memory, Magnetic RAM, etc.), or any other suitable storage media.Such a computer-readable medium may be implemented in any suitablemanner, including as computer-readable storage media 806 of FIG. 3described below (i.e., as a portion of a computing device 800) or as astand-alone, separate storage medium. As used herein, “computer-readablemedia” (also called “computer-readable storage media”) refers totangible storage media. Tangible storage media are non-transitory andhave at least one physical, structural component. In a“computer-readable medium,” as used herein, at least one physical,structural component has at least one physical property that may bealtered in some way during a process of creating the medium withembedded information, a process of recording information thereon, or anyother process of encoding the medium with information. For example, amagnetization state of a portion of a physical structure of acomputer-readable medium may be altered during a recording process.

In some, but not all, implementations in which the techniques may beembodied as computer-executable instructions, these instructions may beexecuted on one or more suitable computing device(s) operating in anysuitable computer system, including the exemplary computer system ofFIG. 3, or one or more computing devices (or one or more processors ofone or more computing devices) may be programmed to execute thecomputer-executable instructions. A computing device or processor may beprogrammed to execute instructions when the instructions are stored in amanner accessible to the computing device or processor, such as in adata store (e.g., an on-chip cache or instruction register, acomputer-readable storage medium accessible via a bus, acomputer-readable storage medium accessible via one or more networks andaccessible by the device/processor, etc.). Functional facilitiescomprising these computer-executable instructions may be integrated withand direct the operation of a single multi-purpose programmable digitalcomputing device, a coordinated system of two or more multi-purposecomputing device sharing processing power and jointly carrying out thetechniques described herein, a single computing device or coordinatedsystem of computing devices (co-located or geographically distributed)dedicated to executing the techniques described herein, one or moreField-Programmable Gate Arrays (FPGAs) for carrying out the techniquesdescribed herein, or any other suitable system.

FIG. 3 illustrates one exemplary implementation of a computing device inthe form of a computing device 800 that may be used in a systemimplementing techniques described herein, although others are possible.It should be appreciated that FIG. 3 is intended neither to be adepiction of necessary components for a computing device to operate inaccordance with the principles described herein, nor a comprehensivedepiction.

Computing device 800 may comprise at least one processor 802, a networkadapter 804, and computer-readable storage media 806. Computing device800 may be, for example, a desktop or laptop personal computer, apersonal digital assistant (PDA), a smart mobile phone, a server, awireless access point or other networking element, or any other suitablecomputing device. Network adapter 804 may be any suitable hardwareand/or software to enable the computing device 800 to communicate wiredand/or wirelessly with any other suitable computing device over anysuitable computing network. The computing network may include wirelessaccess points, switches, routers, gateways, and/or other networkingequipment as well as any suitable wired and/or wireless communicationmedium or media for exchanging data between two or more computers,including the Internet. Computer-readable media 806 may be adapted tostore data to be processed and/or instructions to be executed byprocessor 802. Processor 802 enables processing of data and execution ofinstructions. The data and instructions may be stored on thecomputer-readable storage media 806.

The data and instructions stored on computer-readable storage media 806may comprise computer-executable instructions implementing techniqueswhich operate according to the principles described herein. In theexample of FIG. 3, computer-readable storage media 806 storescomputer-executable instructions implementing various facilities andstoring various information as described above. Computer-readablestorage media 806 may store the various processes/facilities discussedabove.

While not illustrated in FIG. 3, a computing device may additionallyhave one or more components and peripherals, including input and outputdevices. These devices can be used, among other things, to present auser interface. Examples of output devices that can be used to provide auser interface include printers or display screens for visualpresentation of output and speakers or other sound generating devicesfor audible presentation of output. Examples of input devices that canbe used for a user interface include keyboards, and pointing devices,such as mice, touch pads, and digitizing tablets. As another example, acomputing device may receive input information through speechrecognition or in other audible format.

Embodiments have been described where the techniques are implemented incircuitry and/or computer-executable instructions. It should beappreciated that some embodiments may be in the form of a method, ofwhich at least one example has been provided. The acts performed as partof the method may be ordered in any suitable way. Accordingly,embodiments may be constructed in which acts are performed in an orderdifferent than illustrated, which may include performing some actssimultaneously, even though shown as sequential acts in illustrativeembodiments.

Various aspects of the embodiments described above may be used alone, incombination, or in a variety of arrangements not specifically discussedin the embodiments described in the foregoing and is therefore notlimited in its application to the details and arrangement of componentsset forth in the foregoing description or illustrated in the drawings.For example, aspects described in one embodiment may be combined in anymanner with aspects described in other embodiments.

Use of ordinal terms such as “first,” “second,” “third,” etc., in theclaims to modify a claim element does not by itself connote anypriority, precedence, or order of one claim element over another or thetemporal order in which acts of a method are performed, but are usedmerely as labels to distinguish one claim element having a certain namefrom another element having a same name (but for use of the ordinalterm) to distinguish the claim elements.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” “having,” “containing,” “involving,” andvariations thereof herein, is meant to encompass the items listedthereafter and equivalents thereof as well as additional items.

The word “exemplary” is used herein to mean serving as an example,instance, or illustration. Any embodiment, implementation, process,feature, etc. described herein as exemplary should therefore beunderstood to be an illustrative example and should not be understood tobe a preferred or advantageous example unless otherwise indicated.

Having thus described several aspects of at least one embodiment, it isto be appreciated that various alterations, modifications, andimprovements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the spirit and scope ofthe principles described herein. Accordingly, the foregoing descriptionand drawings are by way of example only.

What is claimed:
 1. A system comprising: an extruder including a screw configured to rotate in a barrel to convey a mixture of polymeric material and blowing agent in a downstream direction in a polymer processing space defined between the screw and the barrel; a mold connected to an outlet of the extruder, the screw being configured to periodically move in a downstream direction in the barrel to inject a shot of the mixture of polymeric material and blowing agent into the mold; a hopper configured to hold polymeric material pellets and blowing agent in a chamber volume, the hopper including at least one inlet connectable to a source of the blowing agent, the hopper including an outlet connectable to the polymer processing space in the extruder; a pressure regulator constructed and arranged to control the pressure of blowing agent supplied to the hopper; at least one processor; and at least one storage medium having encoded thereon executable instructions that, when executed by the at least one processor, cause the at least one processor to carry out a method comprising: controlling a pressure of the blowing agent supplied to the hopper to a desired pressure using the pressure regulator based, at least in part, on the desired weight percentage of blowing agent in the shot of the mixture of polymeric material and blowing agent.
 2. The system of claim 1, wherein controlling a pressure of the blowing agent supplied to the hopper to a desired pressure comprises: determining the volume of blowing agent in the hopper.
 3. The system of claim 1, wherein the source of blowing agent comprises nitrogen.
 4. The system of claim 1, wherein the mold comprises an injection mold.
 5. The system of claim 1, wherein the hopper includes multiple chambers.
 6. A method comprising: providing a hopper configured to hold polymeric material pellets; supplying blowing agent to the hopper at a desired blowing agent pressure based, at least in part, on a desired weight percentage of blowing agent in the shot; supplying blowing agent and the polymeric material pellets to an inlet of an extruder including a screw configured to rotate in a barrel; conveying a mixture of polymeric material and the blowing agent in a downstream direction in the extruder; accumulating a shot of the mixture of polymeric material and blowing agent; and injecting the shot into a mold to form a molded polymeric foam article.
 7. The method of claim 6, wherein a pressure regulator is configured to provide the desired blowing agent pressure.
 8. The method of claim 6, further comprising determining the desired blowing agent pressure, in part, from the volume of blowing agent in the hopper.
 9. The method of claim 6, wherein the source of blowing agent comprises nitrogen.
 10. The method of claim 6, wherein the mold comprises an injection mold.
 11. The method of claim 6, wherein the hopper includes multiple chambers.
 12. A system comprising: an extruder including a screw configured to rotate in a barrel to convey a mixture of polymeric material and blowing agent in a downstream direction in a polymer processing space defined between the screw and the barrel; a mold connected to an outlet of the extruder, the screw being configured to periodically move in a downstream direction in the barrel to inject a shot of the mixture of polymeric material and blowing agent into the mold; a blowing agent introduction system including a source of blowing agent and a pressure regulator; and a hopper including a chamber volume having a port fluidly connected to the source of blowing agent, wherein the hopper is configured to hold polymeric material pellets and blowing agent in a chamber volume, the hopper including a first outlet configured to supply polymeric material pellets and blowing agent to the polymer processing space in the extruder; and wherein the system is configured to recycle blowing agent in the chamber volume to a location in the blowing agent introduction system upstream of the pressure regulator.
 13. The system of claim 12, wherein the system is configured to recycle residual blowing agent in the chamber volume after the polymeric material pellets have been supplied to the polymer processing space.
 14. The system of claim 12, wherein the source of blowing agent comprises nitrogen.
 15. The system of claim 12, wherein the mold comprises an injection mold.
 16. The system of claim 12, wherein the hopper includes multiple chambers.
 17. A system comprising: an extruder including a screw configured to rotate in a barrel to convey a mixture of polymeric material and blowing agent in a downstream direction in a polymer processing space defined between the screw and the barrel; a mold connected to an outlet of the extruder, the screw being configured to periodically move in a downstream direction in the barrel to inject a shot of the mixture of polymeric material and blowing agent into the mold; a blowing agent introduction system including a source of blowing agent; and a hopper including at least a first chamber volume and a second chamber volume, at least one of the first or the second chamber volumes configured to connect to the source of blowing agent and to hold polymeric material pellets and blowing agent.
 18. The system of claim 17, wherein the system is configured to selectively supply the blowing agent and polymeric material from either the first chamber volume or the second chamber volume to the polymer processing space in the extruder.
 19. The system of claim 18, wherein the hopper further comprises a third chamber volume configured to connect to an outlet of the first chamber volume and an outlet of the second chamber volume, the third chamber volume including an outlet connected to the polymer processing space in the extruder.
 20. The system of claim 18, wherein the mold comprises an injection mold. 